4.1.3 Alkenes Flashcards
(21 cards)
What is the structure of a C=C bond in alkenes?
A C=C bond in alkenes consists of a π-bond (sideways overlap of adjacent p-orbitals above and below the bonding carbon atoms) and a σ-bond (overlap of orbitals directly between the bonding atoms).
Why is there restricted rotation around a C=C bond?
Restricted rotation occurs due to the presence of the π-bond which locks the atoms in position.
Explain the shape and bond angle around each carbon in a C=C bond.
The shape is trigonal planar with a bond angle of approximately 120° explained by electron pair repulsion between regions of electron density around each carbon.
Define stereoisomers.
Stereoisomers are compounds with the same structural formula but a different arrangement of atoms in space.
What is E/Z isomerism?
E/Z isomerism is a type of stereoisomerism caused by restricted rotation about a C=C bond and the requirement for two different groups to be attached to each carbon of the double bond.
What is cis-trans isomerism?
Cis-trans isomerism is a special case of E/Z isomerism where two substituent groups attached to each carbon atom of the C=C bond are the same.
How do the Cahn–Ingold–Prelog (CIP) priority rules determine E and Z isomers?
The groups attached to each carbon in the C=C bond are assigned priorities based on atomic number; the E isomer has higher-priority groups on opposite sides while the Z isomer has them on the same side.
Why are alkenes more reactive than alkanes?
Alkenes are more reactive due to the relatively low bond enthalpy of the π-bond in the C=C bond.
What is the test for unsaturation in alkenes?
Add bromine water to the sample; if the solution decolourises it indicates the presence of a C=C bond.
What are the products of the reaction of alkenes with hydrogen?
Alkanes are formed when alkenes react with hydrogen in the presence of a suitable catalyst such as nickel (Ni).
What is the product of the reaction of alkenes with halogens?
Dihaloalkanes are formed when alkenes react with halogens.
What is formed when alkenes react with hydrogen halides?
Haloalkanes are formed when alkenes react with hydrogen halides.
What is the product of the reaction of alkenes with steam and an acid catalyst?
Alcohols are formed when alkenes react with steam in the presence of an acid catalyst such as H3PO4.
Define an electrophile.
An electrophile is an electron pair acceptor.
What is the mechanism for the addition reactions of alkenes?
The mechanism is electrophilic addition involving heterolytic fission.
What is Markownikoff’s rule?
Markownikoff’s rule predicts the major product in the addition of H-X to unsymmetrical alkenes based on the relative stabilities of carbocation intermediates (tertiary > secondary > primary).
What is addition polymerisation?
Addition polymerisation involves the joining of many alkene monomers to form a polymer by breaking the π-bond.
How can the repeat unit of an addition polymer be deduced?
The repeat unit can be deduced by identifying the structure of the monomer and removing the double bond.
How can the monomer of an addition polymer be identified?
The monomer can be identified by locating the repeat unit and restoring the double bond in the structure.
What are three benefits of processing waste polymers for sustainability?
Combustion for energy production use as an organic feedstock and removal of toxic waste products such as HCl during combustion of halogenated plastics.
What are two environmental benefits of developing biodegradable and photodegradable polymers?
They reduce dependency on finite resources and alleviate problems associated with the disposal of persistent plastic waste.
